It sounds at first like an electronic whine, a build-up of noise in the amplifier. Then there's Lou Reed's voice, young but hardened: "When the smack begins to flow, and it shoots up the dropper's neck, and I'm rushing on my run, then I feel just like Jesus's son." Behind it all, there's that strange keening, humming note. Listen to the Velvet Underground's Heroin on headphones and you realise it's not feedback after all, not a synthesised warble, but the rich timbre of a violin playing a single note, held for a disturbingly long time.